Photobleaches are often added to detergent laundry powders to provide bleaching of the washed fabrics to improve their visual appeal. The most common and simplest way of incorporating them into detergent powders is to add them to the slurry prior to spray drying of the base powder. However some of the photobleach may be lost due to degradation and evaporation in the spray drying process. Furthermore, there is a well-known problem of staining of the washed fabric due to the photobleach.
EP 119 746A (Unilever) discloses spraying a solution or suspension of a Zn or Al phthalocyanine sulphonate photobleach onto a detergent base powder subsequent to the spray drying process.
WO 99 51714A (Unilever) discloses a coloured photobleach speckle composition giving reduced staining wherein the carrier is an α-hydroxy organic acid, preferably citric acid, with a water-soluble barrier material and a flow aid e.g. silica. The composition is claimed to provide reduced fabric staining.
DE 3 430 773A (Ciba Geigy) discloses a washing powder additive in speckle form which contains a photobleach, an inorganic carbonate and an acid. Effervescence action generated by the reaction between the carbonate and the acid is claimed to keep the speckles at the surface of the wash liquor and reduce staining of the washed fabrics.
U.S. Pat. No. 3,931,037 (Procter & Gamble) discloses a colourless granular composition comprising a pale-coloured photobleach granule made by the dry-mixing of a phthalocyanine photobleach particle and a granular material (e.g. sodium tripolyphosphate or spray-dried detergent base powder) to which a liquid (e.g. water) is added.
U.S. Pat. No. 4,762,636 (Ciba-Geigy) discloses a process for the preparation of spray-dried floating speckles that contain an active substance.